Diverse communities broaden ecosystem function


  • Species occur together
    • not all perfectly adapted
    • networks of trophic interactions


  • Functioning: Individual and collective contributions to energy flow and matter recycling


  • Ecosystems work better as a group
    • No single species performs all roles
    • larger threshold for environmental change
    • fosters ecosystem resilience
      • ball & cup model

Diverse communities broaden ecosystem function




  • Ecosystems work better as a group
    • No single species performs all roles
    • larger threshold of environmental conditions
    • fosters ecosystem resilience
      • ball & cup model

Not all species in a community are equal in importance

Hierarchical levels in an community: Food webs/trophic levels

Energy flow limits length of food chain (10% rule)

Inefficient energy flow limits the number of trophic levels (~3-5 max)

Trophic cascades: Multi-level disruptions in food web

Trophic Cascade: When a trophic level is suppressed, causing changes at one or more other levels

Reintroduction of wolves in Yellowstone: Top down control

Bottom up control of communities: resources limit producers

Building ecosystems: Succession

Disturbances are the major cause of fluctuation in succession


  • Discrete event in time that:
    • removes biomass
    • alters populations & communities
    • alters structure of ecosystems
    • changes resources availability
    • changes physical environment


  • Natural vs Human caused disturbance
    • lightning, herbivory, flood
    • mining, clear-cutting


  • Maintains communities in non-equilibrium
    • ecosystems rarely reach steady-state

Disturbance & succession

Severity of disturbance: intensity, size, and duration


Severity of natural disturbances: frequency


Primary succession




  • After severe disturbances
    • remove or bury biomass
    • no soil or seeds remain


  • Glacial retreat, mud-flows, floodplains, lava flows


  • Initial Colonization:
    • species that handle high stress
    • low nutrients / minimal soil

Primary succession: facilitators



  • Initial Species composition:
    • nitrogen fixers (bacteria, plants)
    • Facilitation:


  • Colonizers must disperse to site
    • wind dispersed seeds/spores
    • algae & lichens
    • small seeded plants
    • distance matters


  • Is primary succession fast or slow?

Secondary succession: soil is key…

Nutrients and seed banks in soil allows succession to occur faster

Secondary succession: pioneer species





  • Potential growth rate:


  • Tolerance:


  • Seed size & Number:


  • Longevity:

Late succession: inhibition & tolerance (shade matters!)

Post disturbance in an Appalachian ecosystem

Study aid: Glacier Bay Alaska succession